1. Field of the Invention
The present invention relates to a backlight unit, and more particularly, to an edge light backlight unit using a light guide panel (LGP) and a point light source.
2. Description of the Related Art
In general, flat panel displays are largely classified into light emitting displays and light receiving displays. Light emitting flat displays include elements such as a cathode ray tube, a plasma display, an electron emitting device, and a fluorescent display.
In comparison, a light receiving type flat panel display device (e.g., a liquid crystal display device) forms an image not by emitting light by itself, but by receiving light from the outside. Thus, it is difficult to view an image on such a display device in dark places. Accordingly, a backlight unit for emitting light is installed at a rear surface of the light receiving type flat panel display device. Such a backlight unit has been used in surface light source devices (e.g., an illuminating signboard), as well as light receiving displays (e.g., a liquid crystal display device).
The backlight unit may be either a direct light backlight unit, or an edge light backlight unit, according to the arrangement of light sources thereon. The direct light backlight provides a plurality of lamps directly under an LCD so as to directly illuminate light on a liquid crystal panel. The edge light backlight unit provides lamps at a sidewall of an LGP in order to illuminate light on a side of the LGP and transmit the light to a liquid crystal panel.
More specifically, the edge light type may use a linear light source or a point light source as a light source. Typically, a cold cathode fluorescent lamp (CCFL) is used as the linear light source, while a light emitting diode (LED) is used as the point light source. The CCFL can emit a strong white light with high brightness and high uniformity, and can enable a large size design of a device. However, the CCFL is disadvantageous since it is operated by a high frequency AC signal and has a narrow operational temperature range. In comparison, the LED exhibits a lower brightness and uniformity than the CCFL, but is operated by a DC signal and has a long life span and a wide operational temperature range. Further, the LED can be manufactured therein.
When an LGP is used in an edge light backlight unit, the light output from the linear or point light source is input to an edge of the backlight unit to be incident upon the LGP. The LGP then converts the incident light into a surface light and outputs it in a vertical (i.e., orthogonal to the LGP surface) direction. The LGP is formed of a material having high light transmittance (e.g., an acryl-based transparent resin such as polymethyl methacrylate (PMMA), or an olefin-based transparent resin). A scattering pattern or a hologram pattern is also formed on the LGP so that light, which is input to an edge of the backlight unit and is incident upon the LGP, is output to a light emitting surface.
FIG. 1 is a perspective view of a related art backlight unit of an edge light type using a point light source. FIG. 2 is a cross-sectional view of the edge light backlight unit of FIG. 1. Referring to FIG. 1, three LEDs 20, which are point light sources, are installed at an edge 11 of an LGP 10. A hologram pattern 30 is formed at a rear surface of the LGP 10 in order to emit light input from the LEDs 20 to a light emitting surface 12.
The LEDs 20 emit light toward the edge 11 of the LGP 10. The LEDs 20 are point light sources. Each LED 20 emits light within an azimuth angle of ±90° with respect to an optical axis 21, as shown in FIG. 3. An azimuth angle, at which the intensity of light is half of the maximum value Imax of the light intensity, is called a radiation angle. In the case of an LED, light is emitted at a radiation angle of ±45°.
The light output from the LCDs 20 is incident upon the LGP, passes through the edge 11, 10, and finally, is incident on the hologram pattern 30. The hologram pattern 30 having a diffraction grid structure formed perpendicular to an optical axis 21 changes the incident light to a surface light and makes the surface light proceed through the light emitting surface 12 which is an upper surface of the light guide panel 10.
The hologram pattern 30 can emit light at the highest efficiency when the light is incident on the hologram pattern 30 at an angle of 90°. Also, as the distribution of an incident azimuth angle of the light incident on the hologram pattern 30 decreases, a uniform brightness can be obtained at the light emitting surface 12. If the brightness of the light emitting surface 12 is not uniform, a screen appears to be smeared.
For example, in a narrow range of about 1 cm, a change in brightness of about 0.9 is detected as a smear. However, when the brightness changes gradually from the central portion of the screen to an edge portion thereof, a change in brightness of about 0.8 is not detected as a smear. Thus, a uniformity of brightness over 0.8 is needed. In particular, to obtain a quality image, a uniformity of brightness over 0.9 is needed.
FIG. 4 is a diagram illustrating the distribution of light output from the conventional backlight unit of FIG. 1. The light guide panel 10 is divided into three portions, that is, a near portion 40, a middle potion 50, and a far portion 60, sequentially from the edge 11 where the LEDs 20 are installed. FIG. 4 also shows the distribution of the output of light, and it can be seen that the middle portion 50 and the far portion 60 have a wider light output distribution compared to the near portion 40.
FIG. 5 is a graph showing the brightness at the light emitting surface 12 of the edge light backlight unit shown in FIG. 1. In the graph, the vertical axis indicates brightness and the horizontal axis indicates FWHM (full width half maximum) showing a light emitting angle at the light emitting surface 12. Three curves C1, C2, and C3 from the left side indicate the brightness of the near portion 40, the middle portion 50, and the far portion 60, respectively. Referring to FIG. 5, it can be seen that the brightness of the near portion 40 is greater than those of the middle portion 50 and the far portion 60. The FWHM of the near portion 40 is 20°/20° while those of the middle portion 50 and the far portion 60 are 20°/35° which are relatively wider than that of the near portion 40. In 20°/35°, the first angle “20°” and the second angle “35°” denote FWHMs in an X direction and a Y direction, respectively.
The irregularity of brightness is caused because the distribution of an incident azimuth angle of the light incident on the hologram pattern 30 is different in each of the near portion 40, the middle portion 50, and the far portion 60. Thus, an efficiency of the light emission by the hologram pattern 30 and the distribution of an emitting azimuth angle of the emitting light are different in the three portions. Generally, the larger the azimuth angle of light input to the LGP 10, the more the brightness of the light emitting surface 12 becomes non-uniform.
Such a problem occurs in an LGP having a scattering pattern as well as an LGP having a hologram pattern.